voltage question

[iknowjohnny]

On merlin's page on PSU's, he suggests using a cap in parallel with the first cap or "resivoir cap" that is 10 time smaller. I use a 100UF there so i would use a 10uf as per his suggestion. The 100uf is 500v and the volatge at that point is something like 450 i think. If i add a 10 uf in parallel with it, whats the lowest voltage cap i can safely get away with? I ask because i doubt i have a 10uf of very high voltage rating, and i want to try this.

[martin manning]

The caps are paralleled, so they will see the same voltage. If you want the same operating margin as the 100uF, it'll have to be a 500V.

[PCollen]

On merlin's page on PSU's, he suggests using a cap in parallel with the first cap or "resivoir cap" that is 10 time smaller. I use a 100UF there so i would use a 10uf as per his suggestion. The 100uf is 500v and the volatge at that point is something like 450 i think. If i add a 10 uf in parallel with it, whats the lowest voltage cap i can safely get away with? I ask because i doubt i have a 10uf of very high voltage rating, and i want to try this.

I haven't read the referenced 'merlin's page', but I must ask: whats the point ? Parallelling 10uF with 100uF will give you 110uF ? What's the point ? Why is 10% (10uF) additional capacitance needed ? Anyway, in your case, you should use a 500V 10uF cap, given that
you have 450V across it and it's in parallel with the 100uF 500V cap. If you use 450V, the 10uF will be the 'weak link in the chain' and will be running at it's maximum voltage rating which is not that good to do.

[iknowjohnny]

From the "smoothing and filtering" section....

ESR: The capacitors used in the power supply are large and so should have low reactance at all frequencies. However, they are not perfect, and actually have an Effective Series Resistance (ESR) and this sets the output impedance of the power supply. Some 'high ripple current' capacitors are designed specially to have a very low ESR, but these are expensive and unnecessary for guitar. Even so, for the power supply you should use the best quality capacitors you can afford, since they are mostly what you end up listening to, because they are where the amp draws its current from most of the time. Replacing old and tired electrolytic capacitors with new ones usually results in a phenomenal improvement bass response and tone.
The reservoir capacitor needs to have a low reactance at frequencies up to 40kHz, because the HT at this point will contain high frequency harmonics caused by rectification. Its performance at these frequencies can be greatly improved by adding a capacitor in parallel with it, ten times smaller in value. (Another can also be added, ten times smaller than the second, but this would be usually be considered an extravogance.)

[Jana]

I sometimes like to put a .022 to .1uf cap in parallel with the 50uf caps in first couple of pre-amp stages.

[David Root]

That's more like the ratio I would use, ie. 1:100 max. This is an old Hi Fi trick.

Jana, what kind of amp uses 50uF on a preamp stage? Other than HiFi.

[iknowjohnny]

Ever try merlin's suggestion? (1st filter)

[Jana]

David,

Marshall used 50uf on many of their amps.

My home brew amps use 50uf on all the stages except for the first two in the power supply rails (OT center tap and screens).

Johhny, if you mean 10:1? No. I'm not discounting it, I just never bothered getting 5 to 10uf poly caps (which is what you should use for this trick). The smaller caps worked fine for me. I wasn't interested in going total hifi, just adding a bit of clarity.

[iknowjohnny]

Well no, i didn't mean that. (size) I meant have u ever done this on the 1st cap, or the plate node.

[aclempoppi]

Would this cap be similar in function to the 8uf/450, used in the "68 Bassman ? Art

[C Moore]

I thought the point of this was not the increased capacitance, which is almost nil, but the use of a non-electrolytic cap. I have been told a poly-film type cap is a better attenuator of high frequencies. Have I been a victim of more INTERNET myth making.....?? Or am I thinking of a different application?

[greenbottle]

Surely the key words are 40kHz and High frequency harmonics. A 10R 10w resister in series in HT line and a 100nf cap of suitable voltage in parrallel is sufficient to reduce rectifier noise. It performs the the same function as the caps around the diodes in the Blues Master cct.

Alan

[iknowjohnny]

Last nite i put mallory .022's across the rectifier diodes. @ diodes per side, and i used 2 mallories in series across each of the 4 diodes to double thier 630 volt handling. But i'd like to put single caps there so I have a couple questions..... if i used one cap per diode would 630v be enough, as i hate having all those doubled up caps? And is there an optimal value to use? I know merlin says .01uf, but if i have to go with 1000v as he states, i doubt i will have a lot of choice as to cap value.

[rp]

Has anyone (Jana?) tried this in a guitar amp and if so what have you heard?

Paralleling the PS caps with a smaller cap is almost standard in modern hifi, but I always read that's it must be a film cap over the electrolytic. 10mf @ 600 film is gonna be awful big - as big as the electrolytic. I too have always seen these spec'd closer to 100:1 not 10:1. Maybe this is moot now that big film caps are available to replace electrolytics? Would you still parallel a 40mf Solen? Maybe check some C-J schematics, I know they only use films in the ps now.

There used to be info on this at Welborne Labs and the Parts Connexion sites.

I never tried it, I only know enough to clone vintage amps, and for me these never seem to benefit from modern tricks. If I could design/voice something new I would try it - especially if building a bass amp. Still for MI amps it's more stuff to fail, more stuff to troubleshoot, more stuff taking up space. The advantages would have to be high.

[Colossal]

Yes, I've tried it. Have done it in Marshally amps and just finished a jazz amp doing it. In fact I paralleled every 22uF electrolytic in that amp with an Xicon MPP polypropylene cap at the 1:100 ratio. The amp is dead quiet but I attribute that to the sum of good filtering, grounding, and lead dress, not just due to the added filtering from the paralleled film cap(s). But I like doing it.

Lots of people naysay this technique and will wage pitched battles over its efficacy. IME though, film caps and even all film cap power supplies will punch up the low end in an amp with an apparent increase in clarity and maybe a slightly faster, punchier response. This is generalizing however and each amp design will differ. Try paralleling an 0.1-0.68uF film cap on an electrolytic in a preamp (especially on V1) and see if you can hear the difference. The Valve Junior guys have been doing this and have dubbed it the Jedi Cap Trick since it was discovered that Ephiphone began adding a paralleled film cap on one of the preamp stages in the Vjr. But a good application is to parallel the reservoir cap with a 1:100 film cap to shunt rectifier noise to ground. I like to use good quality film bypass caps whenever possible as I do notice better bass response and less fuzz.

The 10R with 100nF to ground before the rectifier and reservoir cap is similar to a snubbing circuit and variants have been detailed by Torres et al and are a staple in hi-fi circuits.